Films made from various ethylene interpolymers, viz. ethylene-acrylic acid, ethylene-methacrylic acid and ionomers thereof are well known. Such interpolymers are prepared commercially in high pressure reactors at elevated temperatures in the presence of free radical catalyst. Depending on the conditions of synthesis, the interpolymer may be compositionally uniform and either random or non-random, or it may be compositionally non-uniform.
As described in U.S. Pat. No. 4,248,990, by compositionally uniform it is meant that substantially all of the polymer molecules have substantially the same chemical composition while the molecular weight thereof may vary. Randomness is generally measured by the ratio of percent adjacent interpolymerized acid to total percent interpolymerized acid in the interpolymer. When this ratio is above 0.44, the interpolymer is considered to be non-random, while below 0.44 it is considered to be random.
Also as described in U.S. Pat. No. 4,248,990, the synthesis conditions will determine whether the interpolymerization is one-phase operation or two-phase operation. Generally, as the pressure and temperature of the reactor are reduced, there will be a transition from one-phase operation to two-phase operation evidenced by an increase in initiator demand, development of a significant temperature spread in the reactor, and the requirement of more power to maintain constant agitator speed, while the interpolymer product will exhibit a narrower molecular weight distribution, a decrease in the slope of the shear rate-shear stress curve for the melt, an increase in the environmental stress crack resistance, an increase in the maximum draw rate for film or filament, and an increase in the ratio of percent adjacent acid to percent total acid. The interpolymer produced in two-phase operation will be compositionally non-uniform, while that produced in one-phase operation will be compositionally uniform. In one-phase interpolymerization just above the transition zone, the interpolymer product will be non-random, but will be increasingly random as the pressure and temperature are elevated above the transition zone. At pressures more than 3.3 MPa above and temperatures more than 15.degree. C. above the transition zone, the interpolymer product is substantially random. When the synthesis conditions are at least about 26.7 MPa above and about 20.degree. C. above the transition zone, the interpolymer product exhibits superior physical properties and surprisingly excellent optical properties. These superior random ethylene interpolymers and their method of preparation are disclosed in U.S. Pat. 4,599,392.
Various additives for improving slip and antiblocking characteristics of ethylene interpolymers containing acrylic acid, methacrylic acid and ionomers thereof have been known. Some of these were known before the above described effect of synthesis conditions on interpolymer properties was known. For example, U.S. Pat. No. 3,597,382 described ethylene copolymers containing 2-25 wt.% of an .alpha.,.beta.-ethylenically unsaturated carboxylic acid copolymerized therein and having a melt index of 0.01-50 dg/min, or inorganic salts thereof, in which the slip characteristics were improved by incorporating therein 0.02-1.0 wt.% of a secondary fatty acid amide of the formula R.sub.1 --CO--NH--R wherein R.sub.1 was a mono-olefinically unsaturated hydrocarbon radical having 20-24 carbon atoms and R.sub.2 was a saturated alkyl group having 14-26 carbon atoms. Secondary fatty acid amides in which R.sub.1 was saturated or in which R.sub.2 was mono-olefinically unsaturated were reported to be ineffective in improving the slip characteristics of the ethylene interpolymers into which it was incorporated.
Similarly, U.S. Pat. No. 3,595,827 described ethylene interpolymer compositions containing in addition to the secondary fatty acid amide of U.S. Pat. No. 3,597,382, 0.01-2 wt.% of synthetic amorphous silica having a surface area of 175-400 m.sup.2 /g, a pore size of 80-250 A, at an average particle size of 0.5-8 microns which synergistically improved the slip characteristics of the ethylene interpolymers, and also improved the antiblocking characteristics thereof.
Because of the state of technology extant at that time, it is believed that the ethylene interpolymers employed in the compositions in U.S. Pat. Nos. 3,597,382 and 3,595,827 were compositionally non-uniform, prepared in two-phase operation.